Gas separation is important in many industries for removing undesirable contaminants from a gas stream and for achieving a desired gas composition. For example, natural gas from many gas fields can contain significant levels of H2O, SO2, H2S, CO2, N2, mercaptans, and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases (e.g., H2S and CO2) be removed from natural gas as possible to leave methane as the recovered component. Small increases in recovery of methane can result in significant improvements in process economics and also serve to prevent unwanted resource loss. It is desirable to recover more than 80 vol %, preferably more than 90 vol %, of the methane when detrimental impurities are removed.
Additionally, synthesis gas (syngas) typically requires removal and separation of various components before it can be used in fuel, chemical and power applications because all of these applications have a specification of the exact composition of the syngas required for the process. As produced, syngas can contain at least CO and H2. Other molecular components in syngas can be CH4, CO2, H2S, H2O, N2, and combinations thereof. Minority (or trace) components in the gas can include hydrocarbons, NH3, NOx, and the like, and combinations thereof. In almost all applications, most of the H2S should typically be removed from the syngas before it can be used, and, in many applications, it can be desirable to remove much of the CO2.
Adsorptive gas separation techniques are common in various industries using solid sorbent materials such as activated charcoal or a porous solid oxide such as alumina, silica-alumina, silica, or a crystalline zeolite. Adsorptive separation may be achieved by equilibrium or kinetic mechanisms. A large majority of processes operate through the equilibrium adsorption of the gas mixture where the adsorptive selectivity is primarily based upon differential equilibrium uptake of one or more species based on parameters such as pore size of the adsorbent. Kinetically based separation involves differences in the diffusion rates of different components of the gas mixture and allows different species to be separated regardless of similar equilibrium adsorption parameters.
Adsorptive separation processes may use packed beds of adsorbent particulates or a structured adsorbent bed, such as a monolith, either in the form of one single block or in the form of extrudates with multiple channels or cells, such as a honeycomb structured monolith. In order to prepare structured adsorbent beds (e.g., monolith) for use in gas separation processes, the beds may be coated with the adsorbent material (e.g., zeolites). However, uniform adsorbent coating is difficult to achieve as well as maintained integrity of the coating during use in gas separation processes, such as pressure swing adsorption (PSA) where high gas velocities can result in degradation of the adsorbent coating. Thus, there is a need for methods of coating adsorbent materials that result in uniform adsorbent coatings which can maintain physical integrity through various separation processes.
Porous inorganic solids have found great utility as separation media for industrial application. In particular, mesoporous materials, such as silicas and aluminas, having a periodic arrangement of mesopores are attractive materials for use in adsorption and separation processes due to their uniform and tunable pores, high surface areas and large pore volumes. Such mesoporous materials are known to have large specific surface areas (e.g., 1000 m2/g) and large pore volumes (e.g., 1 cm3/g). For these reasons, such mesoporous materials enable molecules to rapidly diffuse into the pores. Consequently, such mesoporous materials can be useful as large capacity adsorbents. Additionally, such mesoporous organosilicas may be used as a binder along with other adsorbent materials (e.g., zeolites) to form an adsorbent coating for separation processes.
However, mesoporous organosilicas, which may be used as adsorbents and/or binders are conventionally formed by the self-assembly of the silsesquioxane precursor in the presence of a structure directing agent, a porogen and/or a framework element. The precursor is hydrolysable and condenses around the structure directing agent. These materials have been referred to as Periodic Mesoporous Organosilicates (PMOs), due to the presence of periodic arrays of parallel aligned mesoscale channels. For example, Landskron, K., et al. [Science, 302:266-269 (2003)] report the self-assembly of 1,3,5-tris[diethoxysila]cylcohexane [(EtO)2SiCH2]3 in the presence of a base and the structure directing agent, cetyltrimethylammonium bromide to form PMOs that are bridged organosilicas with a periodic mesoporous framework, which consist of SiO3R or SiO2R2 building blocks, where R is a bridging organic group. In PMOs, the organic groups can be homogenously distributed in the pore walls. U.S. Pat. Pub. No. 2012/0059181 reports the preparation of a crystalline hybrid organic-inorganic silicate formed from 1,1,3,3,5,5 hexaethoxy-1,3,5 trisilyl cyclohexane in the presence of NaAlO2 and base. U.S. Patent Application Publication No. 2007/003492 reports preparation of a composition formed from 1,1,3,3,5,5 hexaethoxy-1,3,5 trisilyl cyclohexane in the presence of propylene glycol monomethyl ether.
However, the use of a structure directing agent, such as a surfactant, in the preparation of an organosilica material, requires a complicated, energy intensive process to eliminate the structure directing agent at the end of the preparation process. For example, calcining may be required as well as wastewater disposal steps and associated costs to dispose of the structure directing agent. This limits the ability to scale-up the process for industrial applications.
Therefore, there is a need for improved methods of coating adsorbent materials for gas separation processes using organosilica materials that can be prepared by a method that can be practiced in the absence of a structure directing agent, a porogen or surfactant.